1. Field of the Invention
The present invention relates to a method used in a wireless communications system and related communication device, and more particularly, to a method of handling multimedia broadcast multicast service transmission and reception in a wireless communications system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) for communicating with a plurality of user equipments (UEs) and communicates with a core network including a mobility management entity (MME), serving gateway, etc for NAS (Non Access Stratum) control. A long term evolution-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system, with carrier aggregation and relay deployment. The carrier aggregation, where two or more component carriers are aggregated, allows a UE of the LTE-A system to simultaneously transmit and receive data via multiple carriers, where the UE of the LTE system can only utilize one carrier for data transmission at any time.
The component carriers of the LTE-A system includes three types: backwards compatible carrier, non-backwards compatible carrier and extension carrier. The backwards compatible carrier is a carrier accessible to UEs of all LTE releases, e.g. the UE of release 8, 9, or 10 (Rel-8/9/10). The non-backwards compatible carrier is a carrier not accessible to UEs of earlier LTE releases, but accessible to UEs of the release defining such a carrier. The extension carrier is a carrier that is not operated stand-alone, but must be a part of a component carrier set where at least one of the carriers in the set is a stand-alone-capable carrier.
In the E-UTRAN, multimedia broadcast multicast service (MBMS) can be provided on a frequency layer dedicated to MBMS (a set of cells dedicated to MBMS transmission i.e. a set of “MBMS dedicated cells”) and/or on a frequency layer shared with non-MBMS services (a set of cells supporting both unicast and MBMS transmissions i.e. a set of “Unicast/MBMS mixed cells”). In both cases, a single frequency network mode of operation is possible for MBMS transmission, known as MBSFN. The MBMS reception is possible for the UE in RRC_CONNECTED or RRC_IDLE states.
An MBSFN synchronization area is an area of the network where all eNBs can be synchronized and perform MBSFN transmissions. The MBSFN synchronization area is capable of supporting one or more MBSFN Areas. On a given frequency layer, an eNB can only belong to one MBSFN synchronization area.
An MBSFN area consists of a group of cells within an MBSFN Synchronization Area of a network, which are coordinated to achieve an MBSFN Transmission. Except for the MBSFN Area Reserved Cells, all cells within an MBSFN Area contribute to the MBSFN Transmission and advertise its availability. A cell within an MBSFN Synchronization Area belongs to only one MBSFN Area for all MBMS services.
Subframe allocation reserved for MBSFN transmission can be transferred by a system information block 2 (SIB2), including parameters of the number of subframe for the MBSFN transmission and related radio frame period and offset.
The eNB can allocate MBSFN subframes and signals the MBSFN allocation to the UE in the abovementioned manner only concerning one single component carrier case (e.g. in the LTE system or only one component carrier enabled in a LTE-A UE). However, the LTE-A UE can simultaneously have multiple component carriers enabled and the component carriers may be enabled as different types of component carriers (e.g. backwards, non-backwards or extension types). Thus, the following scenario may occur. A UE having enabled carrier aggregation considers a MBSFN allocation applied to an extension carrier, but the network does not. In this situation, the UE does not receive downlink data (unicast data) in subframes whose subframe times correspond to the MBSFN subframes times in the extension carrier. However, the network does transmit the downlink data in the subframes, and therefore data loss occurs.
In addition, when the UE capable of transmitting and receiving data on m component carriers receives MBMS services on a MBMS dedicated component carrier, how the UE performs data access and deals with the on-going MBMS service reception is not concerned. Furthermore, the network may simultaneously serve UEs of different LTE system versions, e.g. a release 8, 9 or 10 UE (Rel-8/9/10 UE) and therefore UE management for the UEs should be concerned when deploying resources (e.g. cell/component carrier/frequency layer) for the MBMS service. Or the Rel-8 or Rel-9 UE may camp on a cell of an MBMS dedicated frequency due to cell selection or reselection. However, the cell of MBMS dedicated frequency does not support unicast data transmission/reception such that the UE cannot transmit or receive unicast data, e.g. data related to a Voice over Internet Protocol (VoIP) call.